Vane pump having a pressure compensating valve

ABSTRACT

An apparatus ( 10 ) comprises a pump ( 12 ) and a pressure compensating valve ( 94 ). The pump ( 12 ) includes a member ( 22 ) having a surface ( 24 ) defining a pumping chamber. A rotatable rotor ( 30 ) is located in the pumping chamber. The rotor ( 30 ) has circumferentially spaced vane-like members (42) defining pumping pockets ( 48 ) that expand and contract during rotation of the rotor ( 30 ). The pump ( 12 ) has a fluid circuit ( 72 ) providing fluid pressure for biasing the vane-like members ( 42 ) of the rotor ( 30 ) radially toward the surface ( 24 ). The pressure compensating valve ( 94 ) controls fluid flow through an outlet ( 16 ) and also controls the pressure in the fluid circuit ( 72 ). The pressure compensating valve ( 94 ) has an initial condition blocking fluid flow through the outlet ( 16 ) at pump start-up to provide fluid pressure in the fluid circuit ( 72 ) to bias the vane-like members ( 42 ) of the rotor ( 30 ) radially toward the surface ( 24 ).

TECHNICAL FIELD

The present invention relates to a pressure compensating valve for apump. More particularly, the present invention relates to a pressurecompensating valve for a pump for supplying steering fluid to a powersteering mechanism of a vehicle.

BACKGROUND OF THE INVENTION

Vane pumps are used for supplying fluid to a hydraulic motor of a powersteering mechanism. The vane pump includes a rotor that is rotatablewithin a cam ring. The rotor of the pump includes a plurality ofcircumferentially spaced grooves. A vane is carried in each groove. Thevanes extend radially outwardly from the grooves of the rotor toward asurface of the cam ring. Pumping pockets are formed between adjacentvanes. The pumping pockets receive fluid from an inlet port and deliverfluid to a discharge port of the pump.

When the pump is at rest, i.e., the rotor is stationary relative to thecam ring, the vanes may move radially inwardly into the grooves of therotor and away from the surface of the cam ring. When the rotor beginsto rotate and one or more of the vanes of the pump are in a radiallyinward position, the amount of fluid discharged from the pump is lowrelative to pump operation with all of the vanes extended radiallyoutwardly toward the surface of the cam ring.

A hydraulic power steering mechanism requires a minimum flow rate offluid from the pump for proper operation. When the flow rate is belowthe minimum value, the power steering mechanism may be non-responsive toinputs requesting power steering assistance.

A vane pump generally cannot provide a fluid flow sufficient to reachthe minimum flow rate until all of the vanes of the pump move radiallyoutwardly toward the cam ring surface. Thus, the power steeringmechanism may be not sufficiently responsive from pump start-up untilall of the vanes are positioned radially outward toward the cam surface.

Upon start-up of the vehicle, the vane pump is rotated from a restposition to an angular velocity that is equal to the engine idle speed.For example, some commercial truck engines idle at a speed of between600 and 750 rpm.

In some vane pumps used for supplying fluid to a power steeringmechanism, all of the vanes may not move radially outward toward the camring until the pump reaches an angular velocity that is greater than thevehicle engine's idle speed. For example, in some pumps all of the vanesdo not extend radially outwardly toward the cam ring until the rotor ofthe pump rotates at approximately 900 rpm. Thus, the power steeringmechanism in the vehicle having one of these pumps may not besufficiently responsive until the engine speed is increased to about 900rpm. It is desirable to increase the responsiveness of the hydraulicpower steering mechanism and to provide a pump in which all of the vanesmove radially outward toward the cam ring at a pump speed that is wellbelow the vehicle engine's idle speed.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus comprising a pump and apressure compensating valve. The pump has an outlet for supplyingsteering fluid to a power steering mechanism. The pump includes a member(cam ring) having a surface defining a pumping chamber. A rotatablerotor is located in the pumping chamber. The rotor has circumferentiallyspaced vane-like members defining pumping pockets that expand andcontract during rotation of the rotor. The pump has a fluid circuitproviding fluid pressure for biasing the vane-like members of the rotorradially toward the surface defining the pumping chamber. The pressurecompensating valve controls fluid flow through the outlet and alsocontrols the pressure in the fluid circuit. The pressure compensatingvalve has an initial condition blocking fluid flow through the outlet atpump start-up to provide fluid pressure in the fluid circuit to bias thevane-like members of the rotor radially toward the surface defining thepumping chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of an apparatus constructed inaccordance with the present invention;

FIG. 2 is a schematic illustration of a first plate of a vane pump ofthe apparatus of FIG. 1;

FIG. 3 is a schematic illustration of a second plate of the vane pump ofthe apparatus of FIG. 1;

FIG. 4 is a schematic illustration of a portion of the apparatusconstructed in accordance with the present invention; and

FIG. 5 is a graph comparing an operational characteristic of a pumpembodying the present invention with a prior art apparatus and atheoretic apparatus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates an apparatus 10 constructed inaccordance with the present invention. The apparatus 10 may be used forsupplying hydraulic fluid to a hydraulic motor (not shown), via acontrol valve (not shown), of a vehicle power steering mechanism.

The apparatus 10 includes a housing 14, shown schematically in FIG. 1.The housing 14 includes a single outlet 16 for discharging hydraulicfluid from the apparatus 10 toward the power steering mechanism. Thehousing 14 also includes a single return port or inlet 18 for returninghydraulic fluid from the power steering mechanism. A fluid reservoir 20,shown schematically in FIG. 1, is generally located within the housing14. The fluid reservoir 20 supplies fluid to a vane pump 12 of theapparatus 10 and receives fluid returned to the apparatus from the powersteering mechanism.

The vane pump 12 of the apparatus 10 illustrated in FIG. 1 is a balancedrotary vane pump. Vane pumps other than balanced rotary vane pump may beutilized with the present invention. The vane pump 12 includes a camring 22. The cam ring 22 is fixed relative to the housing 14 andincludes a generally elliptical inner surface 24. Two inlet ports 26extend through the cam ring 22 and terminate at the inner surface 24 ofthe cam ring 22. Two discharge ports 28 also extend through the cam ring22 and terminate at the inner surface 24 of the cam ring. Alternatively,the inlet ports 26 and the discharge ports 28 may be located in a platemounted adjacent cam ring 22 of the pump, such as the plate 52 shown inFIG. 3.

A rotor 30 is mounted within the cam ring 22 and is rotatable relativeto the cam ring 22. Specifically, the rotor 30 is connected to an inputshaft 32. The engine (not shown) of the vehicle (not shown) drives theinput shaft 32. Thus, as the engine rate increases, the rate of rotationof the input shaft 32 increases and thus, the rotation rate of the rotor30 increases.

The rotor 30 has a cylindrical outer surface 34 that is coaxial with theinput shaft 32. A plurality of slots or grooves 36 extends into theouter surface 34 of the rotor 30. FIG. 1 shows ten grooves 36, forexample, extending into the outer surface 34 of the rotor 30. The numberof grooves 36 may be other than ten. The grooves 36 arecircumferentially spaced about the outer surface 34 of the rotor 30 andextend along a length of the rotor. Each groove 36 includes a pair ofparallel extending side walls 38 and terminates at an inner wall 40. Animaginary circle (not shown) connecting the inner walls 40 of thegrooves 36 is coaxial with the outer surface 34 of the rotor 30 and theinput shaft 32.

Each groove 36 in the rotor 30 carries a vane 42. Each vane 42 is agenerally flat, elongated plate. Each vane 42 is movable relative to therotor 30 and is sized to slidingly engaging the side walls 38 of theassociated groove 36.

The vanes 42 move radially inwardly, i.e., contract, and radiallyoutwardly, i.e., extend, in the associated grooves 36. An inner surface44 of each vane 42 remains within the associated groove 36, i.e.,radially inward on the outer surface 34 of the rotor 30, during radialmovement of the vane 42. During normal operation of the vane pump 12, anouter surface 46 of each vane 42 contacts the inner surface 24 of thecam ring 22 and slides along the inner surface of the cam ring duringrotation of the rotor 30. Contact refers to the outer surface 46 of eachvane 42 being in close proximity to the inner surface 24 of the cam ring22 and encompasses a fluid film separating the surfaces.

The vane pump 12 includes a plurality of pumping pockets 48. Eachpumping pocket 48 is defined between adjacent vanes 42 and between theouter surface 34 of the rotor 30 and the inner surface 24 of the camring 22. First and second plates 50 and 52, respectively, as will bedescribed in detail below with reference to FIGS. 2 and 3, form twoadditional surfaces that define the pumping pockets 48. During rotationof the rotor 30 within the cam ring 22, the volume of the pumpingpockets 48 varies. As the vanes 42 associated with a pumping pocket 48extend from the rotor 30, the volume of the pumping pocket 48 increases,i.e., the pumping pocket 48 expands. Contrarily, as the vanes 42 of thepumping pocket 48 contract, the volume of the pumping pocket 48decreases, i.e., the pumping pocket 48 contracts.

When the input shaft 32 of the vane pump 12 is rotated, the rotor 30 isrotated relative to the cam ring 22. During normal operation of the vanepump 12, fluid from the reservoir 20 flows through an inlet port 26 andinto a respective pumping pocket 48 of the pump. The fluid flows intothe respective pumping pocket 48 during expansion of the respectivepumping pocket. As the rotor 30 continues to rotate, the respectivepumping pocket 48 begins to contract. When positioned adjacent adischarge port 28, contraction of the respective pumping pocket 48results in the fluid being discharged through the discharge port 28.

The vane pump 12 illustrated in FIG. 1 includes two inlet ports 26 andtwo discharge ports 28. Thus, during a single rotation of the rotor 30,a respective pumping pocket 48 displaces two volumes of fluid from aninlet port 26 to a discharge port 28. As shown schematically in FIG. 1,the two discharge ports 28 connect to a discharge fluid chamber 54. Asingle fluid passage 56 (FIG. 4) extends downstream of the dischargefluid chamber 54 for carrying fluid toward the outlet 16 of theapparatus 10.

The operation of the vane pump 12 described above and referred to as the“normal operation” occurs when all of the vanes 42 of the vane pump 12are positioned with their outer surfaces 46 in contact with the innersurface 24 of the cam ring 22. However, when the vane pump 12 is atrest, i.e., the input shaft 32 is not rotating the rotor 30, some of thevanes 42 of the vane pump 12 may move to a position in which their outersurfaces 46 do not contact the inner surface 24 of the cam ring 22. Forexample, assuming that the vane pump 12 of FIG. 1 is mounted in avehicle so that the ground is located at the bottom of FIG. 1, gravitymay cause the vanes 42 located on an upper side, as viewed in FIG. 1, toslide downwardly into an associated groove 36 and away from the innersurface 24 of the cam ring 22. In addition to gravity, vehiclevibrations and other factors may cause various vanes 42 to move awayfrom the inner surface 24 of the cam ring 22.

When one or more of the vanes 42 of the rotor 30 have moved away fromthe inner surface 24 of the cam ring 22, the fluid within one pumpingpocket 48 in the pump 12 may flow over a vane 42, i.e., between theouter surface 46 of the vane 42 and an inner surface 24 of the cam ring22, and into an adjacent pumping pocket 48. Specifically, as the rotor30 rotates and a pumping pocket 48 begins to contract, only a smallamount of fluid may be forced out of the discharge port 28. As a result,the flow rate of fluid discharged through the discharge ports 28 of thevane pump 12 at a particular pump speed is relatively low when comparedto the flow rate at that pump speed when all of the vanes 42 arecontacting the inner surface 24 of the cam ring 22.

As the rotor 30 of the pump 12 begins to rotate from a rest position,i.e., start-up of the pump, centrifugal force begins to act on the vanes42 to force the vanes into contact with the inner surface 24 of the camring 22. The centrifugal force generally is insufficient to force all ofthe vanes 42 into contact with the cam ring 22 at a pump speedassociated with the vehicle engine's idle speed. Since the centrifugalforce is generally insufficient to move all of the vanes 42 into contactthe inner surface 24 of the cam ring 22, other provisions for forcingthe vanes against the cam ring 22 are provided, as will be describedbelow.

FIG. 2 illustrates a first plate 50 of the vane pump 12. The first plate50 is located adjacent a first side of the rotor 30. FIG. 3 illustratesa second plate 52 of the vane pump 12. The second plate 52 is locatedadjacent a second side of the rotor 30, opposite the first end. As shownin FIG. 3, an aperture 58 extends through the second plate 52 forreceiving the input shaft 32. A seal (not shown) may be located in theaperture 58 for preventing fluid leakage between a surface defining theaperture and the input shaft 32.

With reference to FIG. 2, an annular groove 60 is formed in a surface ofthe first plate 50. The annular groove 60 is coaxial with the inputshaft 32 and has an inner diameter and an outer diameter. In anassembled vane pump 12, the inner diameter of the annular groove 60aligns with the inner walls 40 of the grooves 36 of the rotor 30. Therotor 30 is shown by dotted lines in FIG. 2. The annular groove 60 actsas a fluid conduit, as will be described below.

With reference to FIG. 3, four arcuate grooves, indicated at 62, 64, 66,and 68, are formed in a surface of the second plate 52. The arcuategrooves 62–68 have an inner diameter and an outer diameter. In anassembled vane pump 12, the inner diameter of each arcuate groove 62–68aligns with the inner wall 40 of the grooves 36 of the rotor 30. Therotor 30 is shown by dotted lines in FIG. 3. Each of diametricallyopposed arcuate grooves 64 and 68 includes a fluid port, shownschematically at 70. As is also shown schematically in FIG. 3, arcuategrooves 64 and 68 form a portion of a fluid circuit, indicated generallyat 72.

With reference again to FIG. 1, a fluid pocket 74 is formed in eachgroove 36 of the rotor 30. The inner wall 40 and side walls 38 of thegroove 36 and the inner surface 40 of the associated vane 42 define thefluid pocket 74. As the vane 42 slides radially inwardly and outwardlywithin the groove 36 of the rotor 30, the volume of the respective fluidpocket 74 decreases, i.e., contracts, and increases, i.e., expands.

The annular groove 60 on the first plate 50 is in fluid communicationwith each fluid pocket 74. As one vane 42 on the rotor 30 moves radiallyoutward, another vane 42 moves radially inward. The radially inwardmovement of the vane 42 forces fluid out of the contracting fluid pocket74. The fluid flows into the annular groove 60 of the first plate 50.Simultaneously, fluid from the annular groove 60 flows into an expandingfluid pocket 74 moving a vane 42 radially outward.

Additionally, each fluid pocket 74 of the rotor 30 is in fluidcommunication with at least one arcuate groove 62–68 of the second plate52. Arcuate grooves 62 and 66 act as fluid conduits similar to thefunction of annular groove 60. Arcuate grooves 64 and 68 form portionsof the fluid circuit 72 and communicate fluid to the fluid pockets 74for forcing the vanes 42 radially outwardly toward the cam ring 22.

As the rotor 30 begins to rotate from a rest position, fluid isdischarged into the discharge ports 28 of the vane pump 12, even whenone or more of the vanes 42 have moved radially inwardly out of contactwith the cam ring 22. This discharge fluid increases the fluid pressurewithin the fluid circuit 72. As a result, the fluid pressure in arcuategrooves 64 and 68 of the second plate 52 increases. This increased fluidpressure in arcuate grooves 64 and 68 is communicated into the fluidpockets 74 of the rotor 30 adjacent arcuate grooves 64 and 68. The fluidpressure communicated by arcuate grooves 64 and 68 acts on the innersurfaces 40 of the vanes 42 to force the vanes radially outwardly towardthe inner surface 24 of the cam ring 22. Arcuate grooves 64 and 68 arelocated in positions adjacent portions of the cam ring where the vanes42 move radially outwardly or extend. When all of the vanes 42 arepositioned radially outward toward the inner surface 24 of the cam ring22, normal operation of the vane pump 12, as described above, begins.

With reference again to FIG. 1, the fluid discharged into the dischargeports 28 enters the discharge fluid chamber 54. Fluid passage 56 extendsdownstream of the discharge fluid chamber 54 for communicating fluidtoward the outlet 16 of the apparatus 10. The discharge fluid chamber 54and fluid passage 56 also form portions of the fluid circuit 72.

As shown in FIG. 4, fluid passage 56 terminates in a spool bore 76within the housing 14 of the apparatus 10. The spool bore 76 has agenerally cylindrical inner surface 78 and includes a discharge orifice80 that connects with the outlet 16 of the apparatus 10.

An orifice plug 82 is located in the discharge orifice 80 of the spoolbore 76. Preferably, the orifice plug is press fit into the dischargeorifice 80. The orifice plug 82 includes a flow control orifice 84 forcommunicating fluid from the spool bore 76 to the outlet 16. The outlet16 of the apparatus 10 is shown in FIG. 4 as including internal threads86 for receiving a discharge conduit (not shown).

A radially extending passage 88 in the orifice plug 82 connects the flowcontrol orifice 84 to an axially extending passage 90 formed in thehousing 14 adjacent the spool bore 76. Passage 90 connects to a pressurechamber 92. Pressure chamber 92 connects to the spool bore 76 near anend of the spool bore 76 opposite the outlet 16.

A pressure compensating valve 94 is disposed in the spool bore 76. Thepressure compensating valve 94 includes a valve spool 96 that is movableaxially within the spool bore 76. The valve spool 96 moves as a functionof fluid pressure, as will be described below.

The valve spool 96 includes a generally cylindrical main body portion98. A cylindrical outer surface 100 of the main body portion 98 of thevalve spool 96 includes a number of annular grooves 102, four of whichare shown in FIG. 4. Each annular groove 102 is a balancing oranti-stiction groove. The annular grooves 102 act as a labyrinth seal,balance the pressure around the valve spool 96 to center the valve spoolin the spool bore 76, and prevent the valve spool from sticking to aportion of the spool bore. The outer surface 100 of the main bodyportion 98 of the valve spool 96 also includes an annular bypass groove104.

The main body portion 98 of the valve spool 96 also includes a firstworking surface 106. The first working surface 106 is generally annular.An elongated member 108 extends axially outwardly from the first workingsurface 106 of the main body portion 98 of the valve spool 96. Theelongated member 108 is generally cylindrical and has a diameter that isapproximately one-third of the diameter of the main body portion 98 ofthe valve spool 96. The elongated member 108 terminates opposite themain body portion 98 of the valve spool 96 at an end wall 110.

The main body portion 98 of the valve spool 96 also includes a secondworking surface 112 opposite the first working surface 106. A spring 114acts between a plug member 116 and the second working surface 112 of thevalve spool 96 to bias the valve spool 96 rightward as viewed in FIG. 4.

When placed in the spool bore 76, the valve spool 96 defines first andsecond variable volume fluid chambers 118 and 120, respectively, in thespool bore. The first fluid chamber 118 is defined between the firstworking surface 106 of the valve spool 96 and the orifice plug 82. Thesecond fluid chamber 120 is defined between the second working surface112 of the valve spool 96 and plug member 116. The second fluid chamber120 receives fluid from pressure chamber 92. Since the second fluidchamber 120 is in fluid communication with the outlet 16 of theapparatus 10, fluid pressure in the second fluid chamber 120 isgenerally equal to the fluid pressure at the outlet.

When biased rightward under the force of the spring 114, the end wall110 of the elongated member 108 covers the flow control orifice 84 ofthe orifice plug 82. Thus, the elongated member 108 prevents fluid flowfrom the first fluid chamber 118 into the flow control orifice 84 andtoward the outlet 16 of the apparatus 10. Since the elongated member 108prevents fluid flow through the flow control orifice 84, fluid pressurein the fluid circuit 72 increases during the initial or start-uprotation of the rotor 30 of the pump 12.

When the fluid pressure in the first fluid chamber 118, and thus fluidcircuit 72, exceeds the combined influence of the fluid pressure in thesecond fluid chamber 120 and the spring 114, the valve spool 96 movesleftward, as viewed in FIG. 4. The movement of the valve spool 96 withinthe spool bore 76 is related to a pressure differential between firstfluid chamber 118 and the combined influence of the fluid pressure inthe second fluid chamber 120 and the spring 114. As the valve spool 96moves leftward, the end wall 110 of the elongated member 108 of thevalve spool 96 moves away from the orifice plug 82 and opens fluid flowinto the flow control orifice 84. As the fluid pressure in the firstfluid chamber 118 continues to increase, the valve spool 96 continues tomove leftward. Contrarily, if the fluid pressure in the first fluidchamber 118 decreases, the combined influence of the fluid pressure inthe second fluid chamber 120 and the spring 114 will move the valvespool 96 rightward.

When the pressure within the first fluid chamber 118 increases to apredetermined level, the valve spool 96 of the pressure compensatingvalve 94 moves leftward a distance sufficient to connect the first fluidchamber 118 with a bypass passage (not shown). Fluid flowing into thebypass passage is conducted away from the outlet 16 of the apparatus 10and may be conducted to the reservoir 20 of the vane pump 12.

With reference again to FIG. 4, the pressure compensating valve 94 alsoincludes a pressure relief valve 122. A pocket 124 extends into the mainbody portion 98 of the valve spool 96 from the second working surface112. Internal threads 126 are formed in the pocket 124 near an openinginto the pocket. A radially extending passage (not shown) connects thepocket 124 to the annular bypass groove 104 for communicating fluid inthe pocket to the bypass passage.

The pressure relief valve 122 includes an orifice plate 128 havingexternal threads 130, a spring 132, and a movable actuator 134. Thespring 132 biases the actuator 134 away from an inner wall 136 of thepocket 124. The orifice plate 128 is screwed into the pocket 124 in thevalve spool 96. An orifice 138 extending through the orifice plate 128receives a nose portion 140 of the actuator 134.

Fluid within the second fluid chamber 120 flows through the orifice 138of the orifice plate 128 of the pressure relief valve 122 and acts onthe nose portion 140 of the actuator 134. The nose portion 140 of theactuator 134 prevents fluid flow from the orifice 138 of the orificeplate 128 into the pocket 124 when the biasing pressure of the spring132 is greater than a fluid pressure in second fluid chamber 120. Whenthe fluid pressure in the second fluid chamber 120 increases above thebiasing pressure of the spring 132, the actuator 134 is moved rightward,as viewed in FIG. 4, and fluid flows into the pocket 124. Fluid flowinginto the pocket 124 passes through the radial passage (not shown), intothe annular bypass groove 104, and then into the bypass passage (notshown).

When fluid within the first fluid chamber 118 is prevented from flowinginto the flow control orifice 84, fluid pressure in the first fluidchamber increases. As a result, fluid pressure in fluid circuit 79increases.

As stated above, arcuate grooves 64 and 68 in the second plate 52 of thevane pump 12 form a portion of the fluid circuit 72. As a result, fluidpressure in arcuate grooves 64 and 68 increases as fluid pressure influid circuit 72 increases. The fluid in the arcuate grooves 64 and 68is communicated into the fluid pockets 74 of the rotor 30 and acts onthe inner surfaces 44 of the vanes 42 to force the vanes radiallyoutwardly toward the inner surface 24 of the cam ring 22. By increasingthe fluid pressure in fluid circuit 72, the fluid pressure in the fluidpockets 74 of the rotor 30 increases. As a result, all of the vanes 42of the pump 12 are forced to extend radially outward and contact theinner surface 24 of the cam ring 22 at a lower vane pump speed.

FIG. 5 is a graph comparing an operational characteristic of anapparatus constructed in accordance with the present invention with aprior art apparatus and a theoretic apparatus. FIG. 5 illustrates theflow from the outlet of each apparatus in relation to the pump speed ofthe pump of each apparatus.

The line labeled A in FIG. 5 illustrates the flow from the outlet of atheoretic apparatus as a function of pump speed. In the theoreticapparatus, all of the vanes of the pump are instantaneously extendedradially outwardly toward the cam ring as rotation of the rotor of thepump begins. As line A illustrates, the flow from the theoreticapparatus increases proportionally with pump speed until a designed flowrate, indicated at X, is achieved. When the designed flow rate X isachieved, additional flow produced by the pump of the theoreticapparatus is bypassed so that a constant flow is output from thetheoretic apparatus. Alternatively, the outlet flow from the theoreticapparatus may be decreased as pump speed increases, as is known in theart.

The line labeled B in FIG. 5 is an apparatus 10 constructed inaccordance with the present invention. As illustrated by line B, uponinitial rotation of the rotor 30, i.e., start-up of the pump, no flow isdischarged from the outlet 16 of the apparatus 10. At the point on lineB labeled Y, all of the vanes 42 of the pump 12 have moved radiallyoutwardly toward the cam ring 22 and the fluid pressure in the firstfluid chamber 118 is sufficient to move the valve spool 96 to open flowthrough the flow control orifice 84 to the outlet 16 of the apparatus10. Once all of the vanes 42 have moved radially outward toward the camring 22 and the valve spool 96 opens the flow control orifice 84, theoutlet flow from the apparatus 10 follows the flow of the theoreticapparatus illustrated by line A.

The line labeled C in FIG. 5 is an apparatus of the prior art. Asillustrated by line C, upon start-up of the pump, very little flow isdischarged from the outlet of the prior art apparatus. In fact, the flowrate is so low that it is illustrated as zero in FIG. 5. At the point online C labeled Z, all of the vanes of the pump of the prior artapparatus have moved radially outwardly toward the cam ring. Once all ofthe vanes have moved radially outward toward the cam ring, the apparatusof the prior art follows the flow of the theoretic apparatus illustratedby line A.

As is clear from the graph of FIG. 5, the apparatus 10 constructed inaccordance with the present invention, more closely emulates thetheoretic apparatus. The vanes 42 of the pump 12 of the apparatus 10move radially outwardly toward the cam ring 22 at a much lower pumpspeed than the prior art apparatus. The spacing between point Y andpoint Z in FIG. 5 illustrates this difference. As a result, theapparatus 10 is more likely to provide the flow necessary to operate apower steering mechanism when the vehicle is operating at its engine'sidle speed.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. Suchimprovements, changes and modifications within the skill of the art areintended to be covered by the appended claims.

1. Apparatus comprising: a pump having an outlet for supplying steeringfluid to a power steering mechanism, said pump including a member havinga surface defining a pumping chamber, a rotatable rotor in said pumpingchamber, said rotor having circumferentially spaced vane-like membersdefining fluid pockets which expand and contract during rotation of saidrotor; said pump having a fluid circuit providing fluid pressure to saidfluid pockets for biasing said vane-like members of said rotor radiallytoward said surface; and a pressure compensating valve for controllingfluid flow through said outlet and for controlling the fluid pressure insaid fluid circuit, said pressure compensating valve having an initialcondition blocking fluid flow through said outlet at pump start-up toprovide fluid pressure in said fluid circuit to bias said vane-likemembers of said rotor radially toward said surface.
 2. Apparatus asdefined in claim 1 wherein said pressure compensating valve is actuatedfrom the initial condition to a condition enabling fluid flow from saidoutlet in response to a pressure increase in said fluid circuit actingon said pressure compensating valve.
 3. Apparatus as defined in claim 1wherein said pump includes a plate located adjacent a side of saidrotor, said plate including at least one groove, said at least onegroove forming a portion of said fluid circuit and being in fluidcommunication with a plurality of said fluid pockets.
 4. Apparatus asdefined in claim 3 wherein said at least one groove is an annular groovethat is in fluid communication with all of said fluid pockets. 5.Apparatus as defined in claim 3 wherein said at least one grooveincludes an arcuate groove having a port through which fluid pressure iscommunicated.
 6. Apparatus as defined in claim 1 wherein said pressurecompensating valve includes a valve spool that is movable within a spoolbore, a spring urging said valve spool against an orifice for blockingfluid flow through said outlet.
 7. Apparatus as defined in claim 6wherein said valve spool divides said spool bore into first and secondfluid chambers, said first fluid chamber forming a portion of said fluidcircuit, fluid pressure in said first fluid chamber acting on said valvespool to compress said spring and move said valve spool away from saidorifice for enabling fluid flow through said outlet.
 8. Apparatus asdefined in claim 7 wherein fluid pressure in said second fluid chamberacts on said valve spool to aid said spring in urging said valve spoolagainst said orifice for blocking fluid flow through said outlet. 9.Apparatus as defined in claim 8 wherein said second fluid chamber is influid communication with said outlet, downstream of said orifice. 10.Apparatus as defined in claim 8 wherein said valve spool includes apressure relief valve, said pressure relief valve being actuatable inresponse to a predetermined pressure to direct fluid away from saidsecond fluid chamber and thereby, reduce fluid pressure in said secondfluid chamber.
 11. Apparatus as defined in claim 1 wherein said pressurecompensating valve includes a valve spool that is movable within acylindrical spool bore, said valve spool including a cylindrical bodyportion having a plurality of annular grooves which act to center saidvalve spool within said spool bore.